1. Field of the Invention
The present invention relates to a pressure-sensitive recording paper, and more particularly to an improvement of the mark formation and printability of the pressure-sensitive recording paper.
2. Prior Art
Generally, a pressure-sensitive recording paper consists of a top sheet (CB) of which the back side is coated with microcapsules, containing colorless or pale colored electron donative leuco dyes desolved in an organic solvent (capsule oil), and of a bottom sheet (CF) which has color developing layer containing electron acceptive color developing agent on its front side.
When the top sheet is laid on the bottom sheet and local pressure is applied on them by handwriting (with a ballpoint pen), or with a typewriter, the capsules to which pressure is applied are ruptured, and the oil in the capsules containing leuco dyes is transferred to the color developing layer, and printed mark is formed by color generating reaction of leuco dyes and coreactants.
The color developing sheet mentioned in this invention is a sheet which has the above color developing layer, including the middle paper (CFB) which is coated with color developing agent on the front side and with capsules on the back side, in addition to a bottom sheet (CF) mentioned above.
In recent years, the demand for the pressure-sensitive recording paper has increased very rapidly with systemization of offices, expansion of the information industry and the spread of computers. As a result, the applications for paper have been diversified, and many different properties have been need with regard to the diversified applications of paper. Not only acquiring vivid recorded images, but the improvement of ink setting of the color developing sheets on which mark forms, are also required in order to adapt to high speed printing of normal ink for characters or lines and desensitizing ink. It is necessary for the color developing layer to absorb and set the normal inks or the desensitizing ink in a very short time, in order to adapt to the high speed printing. Also, stickiness of the printed surface or setting off of printed ink to another paper surface can be prevented by improvement of ink setting speed of the color developing sheets, as a result, high speed printing becomes possible. This is especially important in the case of desensitizing printing, wherein it is required to apply larger amounts of ink than in the case of normal ink printing, so that not only an increase of ink absorbing speed of the color developing layer, but also an increase in the amount of ink absorbance are very necessary in order to accelerate the ink setting. Usually the coating color of the color developing sheet contains electron acceptive color developing agent, with the inorganic pigments, such as kaoline, talc and calcium carbonate or organic pigments, such as urea-formaldehyde resins as fillers. Moreover, latexes and one or more sorts of natural or synthetic aqueous polymers are added in order to fix the above mentioned materials on the sheet. Furthermore, if desired, viscosity adjusting agents and pH adjusting agents and so on can be added. The above coating color is coated on the base paper by a coater and dried. For the purpose of color developing ability advance and ink absorption improvement, many methods for transferring capsule oil efficiently to the color developing sheet have been studied, such as adding some amount of fillers which absorb oil very much in the color developing layer. With regard to these fillers, there are disadvantages in that the adhesion of fillers to the sheet decreases as the amount of fillers increases. Therefore, fillers are picked off from the paper surface during printing, and the fillers adhere to the rubber blanket and scum the printing plate. As a result, practical products cannot be obtained.
One method proposed is to use relatively fine calcium carbonate of which 55% of the particle size distribution is less than 2.mu., in Japanese Laid-Open Patent Application No. 28857/1980. However, these finer fillers have a higher specific surface; therefore, the fixing strength will be insufficient, if the usual amount of binder is used, and it becomes impossible to fix the fillers and other materials to the sheets sufficiently. Then the coated materials are easily picked off, and the surface strength of the sheet is insufficient for commercial printing. The surface strength becomes higher, but the mark forming ability at the same time becomes lower. If a larger amount of binder is added as a countermeasure, the desirable result cannot be obtained.